1. Field of the Invention
The present invention relates to components which are micromechanically structured, and in particular to microvalves as are used, for example, in medical engineering or in pneumatics, and to micropumps using such microvalves.
2. Description of Prior Art
The growing demand for miniaturized and integrated systems has recently led to the development of micromechanic structural parts, such as microvalves and micropumps. In order to effect the necessary mechanical actuation of such components, frequent use is made of piezoceramics which contract in the direction of polarization of the piezoceramic upon application of voltage.
To illustrate the operation of such piezoceramics, FIGS. 6a, 6b and 6c show the state of a piezoceramic in three different voltage drive states. The piezoceramic 900, shown in FIGS. 6a to 6c, has metallizations 910a and 910b on two opposing outer surfaces of piezoceramic 900. Metallization 920a and 920b oppose each other in the direction of the direction 920 of polarization of piezoceramic 900, the voltage U applied to same is zero in FIG. 6a, whereas the voltage is positive in FIG. 6b and negative in FIG. 6c. As can be seen, piezoceramic 900 contracts in a direction transverse to the direction of polarization in the case where the voltage U applied is positive, in comparison to the case where the voltage U applied is zero. Due to the flatness of the piezoceramic, which is, for example, 5 μm, in comparison with an edge length of, for example, 5 mm, a contraction in the direction of polarization is small. The application of a voltage U in the opposite direction of the direction of polarization 920 does not lead to an expansion but to a depolarization of piezoceramic 900, as is indicated by the arrow 920 which is reversed in comparison with the arrows 920a and 920b shown in FIGS. 6a and 6b. 
In order to implement the contractive effect of the piezoceramic, as is represented in FIGS. 6a to 6c, in a suitable manner, use is made, for example, as is known, of a combination of a membrane and a piezoceramic fixed to the same, an example of a bending converter obtained from such a combination being shown in FIGS. 7a, 7b and 7c. A bending converter consists of a piezoceramic 900 which is firmly connected, on a main side, to a membrane 930. While FIG. 7a represents the voltage drive state, since the voltage applied to piezoceramic 900 is zero, FIGS. 7b and 7c show those voltage drive states where the voltage applied to piezoceramic 900 is positive and negative, respectively. If ceramic 900 contracts when a voltage which is positive along the direction of polarization is applied, the firm connection of piezoceramic 900 to membrane 930 causes membrane 930 to bend, as is shown by arrows 940 in FIG. 7b. Consequently the contraction of piezoceramic 900 is converted to a stroke of membrane 930 in a direction 940 away from piezoceramic 900 when a positive voltage is applied to piezoceramic 900. Even though an expansion of piezoceramic 900 and, as a consequence, bending of membrane 930 in the opposite direction should be expected when an inverse voltage, i.e. a voltage which is negative in the direction of polarization, is applied, this voltage drive is small and cannot be utilized in a technical manner since it would lead to a depolarization of piezoceramic 900, this being illustrated by the fact that FIG. 7c is crossed out.
Even though the bending converter described with reference to FIGS. 7a to 7c is fast, exhibits low energy consumption, large/high stroke and a strong force, and, in addition, has the advantage, in particular with regard to employment in microfluidics, that it causes the medium to be switched to be separated from the piezoceramic, a drawback of this type of bending converter is that it can only carry out an active movement in the direction of the membrane (downward in FIGS. 7a to 7c) due to the unsymmetrical nature of the piezoeffect as has been described with reference to FIGS. 6a to 6c. An inverse movement (upward) can only be realized by the bending converter if a voltage is applied in the opposite direction of the direction of polarization, which, however, leads to a depolarization of the piezoceramic even at minor field strengths in the opposite direction. Typical depolarization field strengths of piezoceramics are roughly −4000 V/cm.
A known microvalve uses the bending converter described above so as to realize a valve function wherein the valve is normally open. Such a known normally-open microvalve (in the following referred to as NO valve) is shown in FIGS. 8a and 8b, FIG. 8a representing the closed state of the valve and FIG. 8b representing the normally-open state of the valve. As is shown in FIGS. 8a and 8b, such a conventional NO valve includes a bending converter such as has been described above, which consists of a piezoceramic 900 and a membrane 930, as well as a valve seat arranged below membrane 930 and comprised of a sealing lip 960 which surrounds an opening 970. As is shown in FIG. 8b, in the normally-open case, i.e. if no voltage is applied to the piezoceramic, membrane 930 is spaced apart from sealing lip 960 so that, as is shown by an arrow 980 in FIG. 8b, a fluid may penetrate through opening 970. If a positive voltage is applied to piezoceramic 900, membrane 930 moves, due to the bending as has been explained with reference to FIGS. 7a to 7c, in the direction of sealing lip 960, with the membrane 930 resting, in the fully closed position, on sealing lip 960 so as to close opening 970.
One drawback of the NO valve described above is that, if the voltage applied to the piezoceramic is switched off or interrupted, the membrane returns to its resting position where it is spaced apart from the valve seat, whereby the valve enters into an open state. However, many areas of application, such as medicine, require valves which are closed in their normal state. In drug administration, it must be ensured, for example, that no drug is administered to the patient in the case of a power failure, so as to avoid that the patient is administered an overdose. To prevent this, a “normally closed” function is required.